Panel padding and icons

It's now possible to set the padding of the panel, additionally to the font size. This new option is named padding, the former padding option to reserve screenspace for e.g. additional panels was renamed to strut.

Also view buttons can now contain a pixmap icon, it basically uses the rendering of sublets and has therefore only a tiny overhead for users that don't need it. Views with icons can be defined in a similar way like properties in tags: